The present invention relates to a bistable multivibrator circuit which can be monolithically integrated and has an output that can be placed in a preferential state. The circuit is applicable, for example, to control circuits for high-speed printers and in advanced electronic fuel injection systems for automobile engines.
Bistable multivibrators are known to have particular sequential logic networks characterized by two possible internal stable states (conventionally indicated by the symbols "0" and "1") with which are associated two different states of output or outputs represented by output variables that can be in either of the two states 0 and 1.
The internal state and the state of the outputs of a bistable multivibrator vary in accordance with the input or inputs with which are associated input variables which represent the state thereof and which can only assume the values 1 and 0.
The various types of bistable multivibrators can differ from one another in the number of inputs and in the mode in which the state of the bistable is determined by the configuration of the input states, or by the particular logic function characterizing the bistable.
Thus, the circuitry for each type of bistable can be produced by use of different technologies and base components, while maintaining its own particular logic function.
An electric component particularly adapted for use in logic circuits is the transistor. A transistor, suitably biased, can in fact be driven alternately from a high-voltage, low-current state to a low-voltage, high-current state.
In the first state, between the emitter and the collector terminals, a transistor is practically an open circuit ("off" state or "0" state); in the second state, the transistor is a short circuit ("on" state or "1" state). Thus, the collector-emitter voltage can be adopted as an output variable, associating therewith the values 0 and 1 corresponding to the two different states of the transistor mentioned above, in accordance with a "positive" logic.
The operating mode of the transistor which is the closest to the operation of an ideal switch (with an "off" state and an "on" state) is that in which the transistor, when closed, operates at saturation and is cut off when open. The transistor can be driven to the two different states of saturation and cut-off by appropriately varying the base-emitter voltage; the base-emitter voltage can thus be adopted as an input variable.
The base-emitter levels determining the saturation conditions are higher than those determining the cut off conditions and are quite different therefrom. Thus, one can also associate--with the same "positive" logic adopted for the output variables--the values 1 and 0 with the input variable (base-emitter voltage), respectively, in the case of saturation (a high base-emitter voltage) and in the case of cut-off (a low base-emitter voltage).
The type of bistable that can be produced in the easiest and most economical way from the circuitry point of view is the "S-R" type, having two inputs indicated by the letters S (SET) and R (RESET) and an output whose state corresponds directly to the internal state.
If both input variables of an S-R bistable assume the value 0, the internal state remains unchanged.
If the input S assumes the value 0 and the input R assumes the value 1, the bistable is placed in the "0" state, which corresponds to the value "0" at the output independently of the previous state, and if the input S assumes the value 1 and the input R assumes the value 0, the bistable is placed in the "1" state, which corresponds to the value 1 at the output, independently of the previous state.
The condition where the inputs simultaneously have the value "1" does not determine the state of the bistable.
It is well known that a bistable S-R multivibrator can be realized circuitwise with two main transistors, each having its collector connected through suitable resistors to the base of the other transistor, and to a first pole of a DC supply voltage and having two other auxiliary transistors whose collectors are respectively connected to the bases of the two main transistors and whose emitters are connected to a second pole of the DC supply voltage.
In current technical applications such as, for example, in circuits for controlling high-speed printers or in electronic fuel injection systems for automobile engines, bistable multivibrator circuits are required which can be integrated monolithically and mass-produced economically, such as those of the S-R type, but with an output which can also be placed in a preferential state, independently of the values assumed by the input variables which determine the state thereof under normal operating conditions.